A pulse width modulation (PWM) ampliefier is configured so that a high-frequency triangular wave carrier signal is modulated with an analog signal, for example, an audio signal or the like, so as to be converted into a pulse width signal. The pulse width signal is power-amplified, and then demodulated by removing the carrier signal through a filter immediately before the pulse width signal is applied to a load, such as a speaker or the like. Recently, the foregoing PWM amplifier has been used in mobile audio equipment or the like because of the excellent efficiency of the PWM amplifier in a power amplifying operation.
FIG. 5 shows an example of a conventional PWM amplifier in which an input terminal 1 receives an analog signal. The analog signal is applied to an inverted input terminal of a comparator 2. Simultaneously, the high-frequency output (for example, 200 kHz) of a triangular wave carrier oscillator 3 is applied to a non-inverted input terminal of the comparator 2. Thus, the carrier signal is modulated with the analog signal so as to be converted into a pulse width signal. After being applied through a drive amplifier 4, the pulse width signal obtained from the comparator 2 is amplified by a pulse amplifier (power amplifier) 5 constituted by N-channel MOS power FETs. After the carrier signal is separated from the pulse width signal by a filter circuit which includes a choke coil 6 and a capacitor 7, the demodulated signal actuates, for example, a speaker 9 or the like connected to an output terminal 8.
FIG. 6 shows a conventional pulse width modulation amplifier circuit using a pair of such PWM amplifiers as described above to form a BTL circuit.
In this BTL circuit, an analog signal applied to an input terminal 1 is converted by a differential circuit 10 or the like into first and second analog signals which are different in phase from each other by 180 degrees. Then, the first analog signal is converted by a first comparator 2 into a pulse width signal, which is in turn applied to a choke coil 6 through a drive amplifier 4 and a pulse amplifier 5. Thereafter, the first signal applied to the choke coil 6 is applied to an output terminal 8 after a carrier of the signal has been removed by the choke coil 6 and a capacitor 7 (the filter elements).
Similarly, the second analog signal is converted by a second comparator 2' into a pulse width signal, which in turn is applied to a choke coil 6' through a drive amplifier 4' and a pulse amplifier 5'. Then, the second signal applied to the choke coil 6' is applied to an output terminal 8' after a carrier of the signal has been removed by the filter consisting of a choke coil 6' and a capacitor 7'.
A load 9, for example, a speaker or the like, is connected at its opposite ends to the output terminals 8 and 9, respectively. Therefore, the demodulated analog outputs produced by the PWM amplifiers are applied to the load 9 in an antiphase relation. As such, a voltage across the opposite ends of the load 9 is twice as much as an output voltage obtained by one PWM amplifier, so that, theoretically, electric power which is four times larger than when using one PWM amplifier can be supplied to the load 9.
In the foregoing BTL-PWM amplifier, a triangular wave carrier signal d from one oscillator 3 is applied commonly to the respective comparators 2 and 2' of the two PWM amplifiers so that an analog signal is pulse-width modulated by the comparators 2 and 2'.
The output of the common oscillator 3 is applied, as the triangular wave carrier signal d, to the two PWM amplifiers in the foregoing manner based upon the generation of pulses and the cost factor.
Therefore, when both the input analog signals a and a' to the two PWM amplifiers are zero, square wave pulse outputs b and b' of the pulse amplifiers 5 and 5' are in phase so that they increase and decrease simultaneously as shown in FIGS. 3(a) and 3(b). When the analog signals a and a' achieve a predetermined level, the pulse outputs of the pulse amplifiers 5 and 5' are modulated so that the respective duty factors of the pulse outputs are changed according to the analog signals, as shown in FIG. 3(c). In this situation, the linearity of the triangular wave carrier signal to be applied to the comparators 2 and 2' must be maintained. Hence, if the linearity of the triangular wave deteriorates, an error is generated in converting a voltage level into a pulse width, thereby causing distortion. Therefore, an oscillator 3 having satisfactory performance generally is used.
However, when each of the pulse amplifiers 5 and 5' switches high electric power according to the input pulse, a sharp change in the power supply current results so that power supply voltage also is affected. Because of this change in the power supply voltage, noise spikes sometimes are superimposed on the output waveform of the oscillator 3. As a result, the linearity of the triangular wave carrier signal generated from the oscillator 3 is reduced, so as to reduce the distortion generated.
In the BTL-PWM amplifier of FIG. 6, the analog signals a and a' to be applied to the first and second comparators 2 and 2', respectively, have a phase difference of 180 degrees therebetween. Therefore, the first and second pulse width signals produced from the respective comparators 2 and 2, change in opposite phase.
Consequently, when switching is performed in the pulse amplifier 5 or 5' to correspond to one of the analog signals a or a' and a noise spike thereby is generated, the other analog signal a or a' is compared in the comparator 2 or 2'. As a result, the linearity of the triangular wave carrier signal applied to the comparator 2 or 2' at this time is distorted by the spike noise. Therefore, an error is generated in converting a voltage to a pulse width in the comparator 2 or 2,, so that the distortion worsens.
The present invention has been developed in view of the foregoing problems peculiar to the conventional BTL-PWM amplifier, and is designed to prevent distortion which may be caused when the linearity of a triangular wave applied to one PWM amplifier is deteriorated by a spike noise generated during the switching of the other PWM amplifier.